1. Field of the Invention
The present invention relates to the field of lithium batteries, particularly to materials for anodes of lithium ion batteries.
2. Background Art
Lithium ion batteries have been the focal point of much research due to their higher energy density. Lithium ion batteries have been successfully used in various applications, such as hybrid electric vehicles, mobile electric applications, and renewable energy storage devices. One of the key safety issues in lithium ion batteries would be the dendritic lithium growth on the anode surface at high charging currents because the conventional carbonous materials approach almost 0 V vs. Li+/Li at the end of Li ion insertion.
Li4Ti5O12 has received considerable interest as an anode material for secondary rechargeable lithium-ion batteries. Spinel structure Li4Ti5O12 consists of eight subcells and each subcell has full oxygen atoms, four octahedral interstices and eight tetrahedral interstices. In each elementary cell, two octahedral sites are filled with Li and Ti atoms in a ratio of 0.33:1.66 and one tetrahedral site is filled with one Li atom. Three Li ions can be inserted into the structure at approximately 1.5V vs. Li+/Li. The insertion and extraction of lithium ions is thus a two-phase reaction:Li4Ti5O12+3Li++3e−Li7Ti5O12 
Compared to the conventional carbonous materials, Li4Ti5O12 has been viewed as a promising alternative material for negative electrodes of lithium-ion batteries, because it has several outstanding advantages. These advantages, for example, include: (i) The Li ion insertion into the cubic Li4Ti5O12 spinel structure occurs with little change in the lattice parameter. Consequently, it has near zero volume change (i.e., a zero-strain insertion material) during the charge/discharge processes, which enables an outstanding cycling stability; (ii) High insertion potential of 1.55V (versus Li+/Li), which ensures high safety with electrolyte solution and also non-lithium plating; (iii) Three-dimensional lithium ion diffusion channels producing excellent lithium-ion mobility, which favors lithium-ion batteries charging/discharging fleetly; and (iv) Sharp end-of-charge and end-of-discharge indicators, which are useful for controlling cell operation and preventing overcharge and overdischarge.
Despite the advantages mentioned above, however, pristine Li4Ti5O12 cannot meet the need of practical applications under high current conditions due to its poor electric conductivity, which leads to its low rate capacity. Many efforts have been devoted to improving and optimizing the conductivities of Li4Ti5O12. These efforts include: (1) synthesis of nano-sized particles, because small particle sizes will shorten lithium-ion diffusion paths and broaden the electrode/electrolyte contact surfaces; (2) replacing Li or Ti with other metal cations (i.e., spiking with other metal cations), which will cause a transition from Ti4+ to Ti3+ as charge compensation. The transition will lead to an increase in the electronic conductivity; and (3) adding a second conductive phase into the Li4Ti5O12, such as carbon and conductive oxide.
For example, in “High-performance Li4Ti5-xVxO12 (0≦x≦0.3) as an anode material for secondary lithium-ion battery,” Electrochimica Acta, 54: 7464-7470 (2009), T.-F. Yi et al. disclosed that powders of spinel Li4Ti5-xVxO12 (0≦x≦0.3) synthesized by solid-state methods. Among these materials, Li4Ti4.95V0.05O12 has the highest initial discharge capacity and cycling performance between 1.0 and 2.0V, while Li4Ti4.9V0.1O12 has the highest initial discharge capacity and cycling performance between 0.0 and 2.0V or between 0.5 and 2.0V. The Li4Ti4.9V0.1O12 sample has a good reversibility and its structure is very advantageous for the transportation of lithium-ions.
In “Graphene as a conductive additive to enhance the high-rate capabilities of electrospun Li4Ti5O12 for lithium-ion batteries,” Electrochimica Acta, 55: 5813-5818 (2010), Zhu et al. disclosed versatile electrospinning methods, by which Li4Ti5O12 was processed into nano-sized architectures to shorten the distances for Li-ion and electron transport. Graphene was chosen as an effective carbon coating to improve the surface conductivity of the nanocomposites. The as-prepared graphene-embedded Li4Ti5O12 anode material showed improved discharging/charging and cycling properties, particularly at high rates, such as 22 C. These properties make the nanocomposites attractive anode materials for applications in electric vehicles.
U.S. Pat. No. 6,221,531, issued to Vaughey et al., describes a structure of the spinel type with a general formula Li[Ti1.67Li0.33-yMy]O4, wherein 0≦y≦0.33 and M represents magnesium and/or aluminum. This structure is presented as useful for improving the electronic conductivity of Li4Ti5O12 phase. However, improving electronic conductivity of Li4Ti5O12 phase will not enhance its charging rate capabilities as the electronic conductivity of Li4Ti5O12 anode is important only during the discharging process.
U.S. Pat. No. 6,827,921B1, issued to Singhal et al., describes ultrafine powders of Li4Ti5O12 with particle sizes in the range of 25-500 nm. The average size of particles is about 300 nm or less. The particles are composed of nanocrystallites, which have an average size about 30 nm. The nanostructured (or ultrafine) Li4Ti5O12 powers with a spinel-type structure have improved Li-ion diffusion.
Although these modifications of Li4Ti5O12 have been reported to have improved properties, there are still great challenges to improve the high-rate capability of Li4Ti5O12.